A bolt of lightning usually begins when an invisible negatively
charged stepped leader stroke is sent out from the cloud. As it
does so, a positively charged streamer is usually sent out from
the positively charged ground or cloud. When the leader and streamer
meet, the electric current greatly increases. The region of high
current propagates back up the streamer into the cloud. This "return
stroke" is the most luminous part of the strike, and is the
part that is really visible. Most lightning strikes usually last
about a quarter of a second. Sometimes several strokes will travel
up and down the same leader strike, causing a flickering effect.
Thunder is caused when the discharge rapidly superheats the air
around the strike, causing a shock wave to be sent out.

Positive lightning
makes up less than 5% of all lightning. It occurs when the stepped
leader forms at the positively charged cloud tops, with the consequence
that a negatively charged streamer issues from the ground. The
overall effect is a discharge of positive charges to the ground.
Research carried out after the discovery of positive lightning
in the 1970s showed that positive lightning bolts are typically
six to ten times more powerful than negative bolts, last around
ten times longer, and can strike several miles or kilometers distant
from the clouds. During a positive lighting strike, huge quantities
of ELF and VLF radio waves are generated.